WEBVTT 1 00:00:01.520 --> 00:00:05.599 Welcome to the quarre Side Quantum Physics Podcast, an exploration 2 00:00:05.679 --> 00:00:10.880 of the fundamental structure of reality. We're quantum laws govern matter, energy, 3 00:00:11.199 --> 00:00:16.239 and information. Here, uncertainty is a feature, not a flaw, 4 00:00:16.719 --> 00:00:19.239 and understanding begins at the smallest scales. 5 00:00:27.600 --> 00:00:29.800 Welcome back today. We are going to try to wrap 6 00:00:29.800 --> 00:00:34.119 our heads around something that is simultaneously incredibly small and 7 00:00:34.399 --> 00:00:37.079 absolutely massive in terms of its implications. 8 00:00:37.240 --> 00:00:38.079 That's a good way to put it. 9 00:00:38.119 --> 00:00:40.200 We're talking about the future of computing, but you know, 10 00:00:40.399 --> 00:00:43.280 not the shiny marketing brochure version of it. 11 00:00:43.399 --> 00:00:47.159 No, we are getting into the messy reality, the plumbing 12 00:00:47.240 --> 00:00:48.880 behind the palace, so to speak. 13 00:00:49.240 --> 00:00:52.000 We always hear the grand promise, right We hear that 14 00:00:52.600 --> 00:00:56.520 quantum computers are going to solve these impossible problems revolutionized medicine, 15 00:00:56.799 --> 00:00:59.679 crack encryption codes that would take a normal computer a 16 00:00:59.719 --> 00:01:03.560 billion years to solve. It's painted is this sci fi 17 00:01:03.679 --> 00:01:07.159 utopia where the machine just knows the answer. 18 00:01:07.280 --> 00:01:10.359 It is the ultimate promise of computational power. It's elaked 19 00:01:10.439 --> 00:01:12.680 not just a step. But the gap between that promise 20 00:01:12.719 --> 00:01:15.120 and where we are today is well, it's filled with 21 00:01:15.159 --> 00:01:18.040 some of the most difficult engineering problems in human history. 22 00:01:18.040 --> 00:01:20.239 And today, we are going to zoom in on one 23 00:01:20.280 --> 00:01:21.599 of those problems. 24 00:01:21.319 --> 00:01:24.040 Way way in, right down to the component. 25 00:01:23.680 --> 00:01:27.359 Level, past the headlines, past the hype, and right down 26 00:01:27.400 --> 00:01:30.000 to the microscopic reality of what it actually takes to 27 00:01:30.040 --> 00:01:32.959 build one of these machines. Because it turns out building 28 00:01:32.959 --> 00:01:36.640 the future isn't just about big ideas. Wow, It's about 29 00:01:36.680 --> 00:01:41.359 controlling individual electrons in spaces that are only tens of 30 00:01:41.439 --> 00:01:42.359 nanometers wide. 31 00:01:42.680 --> 00:01:44.719 It is an engineering challenge as much as it is 32 00:01:44.760 --> 00:01:48.760 a physics challenge. You are you're literally wrestling with nature 33 00:01:48.879 --> 00:01:52.359 at its most fundamental level. Howso well, you're trying to 34 00:01:52.480 --> 00:01:56.959 force sub atomic particles to behave against their natural chaotic instincts, 35 00:01:57.319 --> 00:01:59.799 to hold them still and get them to do useful 36 00:01:59.840 --> 00:02:00.439 work for you. 37 00:02:00.760 --> 00:02:02.920 And we have a specific tension to look at today. 38 00:02:02.959 --> 00:02:05.959 We want to pack these things called quantum dots closer 39 00:02:05.959 --> 00:02:08.960 and closer together to build powerful computers, because a computer 40 00:02:09.039 --> 00:02:11.120 with two bits isn't very useful, not at all. But 41 00:02:11.240 --> 00:02:14.120 new research shows that these tiny neighbors are starting to 42 00:02:14.400 --> 00:02:15.759 well shout over each other. 43 00:02:15.879 --> 00:02:17.960 Shouting is a very vivid way to put it. In 44 00:02:18.000 --> 00:02:20.800 the quantum world, shouting is more about energy exchange and 45 00:02:20.840 --> 00:02:26.360 frequency shows but the effect is the same distraction, interference noise. 46 00:02:26.840 --> 00:02:30.199 And we're basing today's discussion on a brand news study 47 00:02:30.240 --> 00:02:33.280 who was published just yesterday February twelve, twenty twenty six, 48 00:02:33.599 --> 00:02:34.960 and physical review applied. 49 00:02:35.120 --> 00:02:36.919 Yes, this is coming out of the Reichen Center for 50 00:02:37.000 --> 00:02:40.280 Quantum Computing. The research was led by Takashi Kobyashi and 51 00:02:40.319 --> 00:02:40.719 his team. 52 00:02:40.879 --> 00:02:42.400 And Reichen is a big deal in this space. 53 00:02:42.639 --> 00:02:45.520 Oh absolutely. Reraichen is a heavyweight in this field. So 54 00:02:45.719 --> 00:02:49.080 when they publish a paper on noise mechanisms, the community, 55 00:02:49.240 --> 00:02:50.439 you know it, sits up and listens. 56 00:02:50.919 --> 00:02:53.319 So what is our mission here? We aren't looking at 57 00:02:53.319 --> 00:02:56.719 a breakthrough where everything works perfectly. We are looking at 58 00:02:56.759 --> 00:02:59.879 a hurdle that was just discovered or at least just 59 00:03:00.080 --> 00:03:01.080 precisely measured. 60 00:03:01.159 --> 00:03:01.960 That's the key. 61 00:03:02.159 --> 00:03:05.400 The goal is to understand what happens when quantum bits 62 00:03:05.479 --> 00:03:08.080 or quibbits get too close for comfort. 63 00:03:08.240 --> 00:03:11.360 It's a story about the growing pains of technology. To 64 00:03:11.439 --> 00:03:14.240 solve a problem, you first have to measure it precisely. 65 00:03:14.960 --> 00:03:17.280 That is what Reichen has done. They haven't just found 66 00:03:17.280 --> 00:03:20.240 a problem, They've put a number on it, They've quantified it. 67 00:03:20.639 --> 00:03:23.759 So we're building the supercomputer of the future. But the 68 00:03:23.840 --> 00:03:25.680 parts are arguing with each other. 69 00:03:25.719 --> 00:03:29.919 In a manner of speaking. Yes, but understanding that argument 70 00:03:30.240 --> 00:03:33.319 is the key to silencing it, or perhaps, as we'll 71 00:03:33.319 --> 00:03:34.800 see later, even using it. 72 00:03:35.000 --> 00:03:37.439 Okay, let's get into the nuts and bolts stand Before 73 00:03:37.479 --> 00:03:39.159 we can really talk about the noise, we need to 74 00:03:39.240 --> 00:03:43.879 understand the instrument. The study focuses on something called quantum dots. 75 00:03:44.639 --> 00:03:47.240 Now I see that term, and my first thought is 76 00:03:47.280 --> 00:03:50.360 those high end TV screens with the really vibrant colors. 77 00:03:50.599 --> 00:03:52.039 Are we talking about the same thing. 78 00:03:52.439 --> 00:03:55.120 That's a great question, and it's a common point of confusion. 79 00:03:55.560 --> 00:03:58.479 It's the same underlying physics, yes, but for a completely 80 00:03:58.520 --> 00:03:59.400 different application. 81 00:03:59.520 --> 00:03:59.840 Okay. 82 00:04:00.039 --> 00:04:04.000 In a TV a quantum dot is a semiconductor nanocrystal 83 00:04:04.240 --> 00:04:08.639 that's designed to glow a very specific pure color when 84 00:04:08.639 --> 00:04:11.159 you hit it with light or electricity. It's an emitter. 85 00:04:11.360 --> 00:04:13.080 It makes pretty colors, it makes very. 86 00:04:12.960 --> 00:04:16.680 Pretty, very precise colors. But in a quantum computer, a 87 00:04:16.800 --> 00:04:18.199 quantum dot is a trap. 88 00:04:18.399 --> 00:04:18.879 A trap. 89 00:04:18.959 --> 00:04:22.120 Think of it as a potential, well, a tiny, tiny 90 00:04:22.199 --> 00:04:25.680 region in a semiconductor material where we can confine a 91 00:04:25.720 --> 00:04:29.040 single charged particle, in this case a single electron. 92 00:04:29.160 --> 00:04:31.439 So you're catching one electron and holding it in place. 93 00:04:31.519 --> 00:04:32.399 That's the entire goal. 94 00:04:32.639 --> 00:04:36.879 Okay, so why silicon. I feel like when people see 95 00:04:37.000 --> 00:04:41.800 pictures of quantum computers, they see those beautiful, big, golden chandeliers. 96 00:04:42.120 --> 00:04:46.319 The superconducting quantum computers from Google or IBM, those don't 97 00:04:46.360 --> 00:04:49.639 look like chips. But Reichen is using silicon. 98 00:04:50.120 --> 00:04:52.879 That is a crucial distinction, and it really defines the 99 00:04:52.920 --> 00:04:58.040 different competing approaches. The chandelier computers use superconducting circuits. Those 100 00:04:58.040 --> 00:05:02.560 are in electronic terms, quite lo large. They're macroscopic loops 101 00:05:02.600 --> 00:05:05.519 of metal cooled to extreme temperatures. 102 00:05:05.600 --> 00:05:07.600 And what Ryichan is doing well, Reichen. 103 00:05:07.360 --> 00:05:10.639 Is working on what's known as silicon's spin quibits. Looks 104 00:05:10.720 --> 00:05:13.279 much more like the chips you'd find in your laptop. 105 00:05:12.920 --> 00:05:15.199 Or your phone. And the advantage of that is the. 106 00:05:15.160 --> 00:05:17.759 Hope is that because we already have, you know, fifty 107 00:05:17.839 --> 00:05:21.199 years of experience and trillions of dollars of infrastructure for 108 00:05:21.319 --> 00:05:24.800 manufacturing billions of transistors on a silicon wafer, right the 109 00:05:24.839 --> 00:05:28.360 whole semiconductor industry exactly, the hope is we can eventually 110 00:05:28.439 --> 00:05:31.800 leverage all of that to manufacture billions of kribbits the 111 00:05:31.839 --> 00:05:32.319 same way. 112 00:05:32.480 --> 00:05:34.920 So it's a bed on scalability. If we can make 113 00:05:34.959 --> 00:05:37.360 it work in silicon, we can make millions of them 114 00:05:37.480 --> 00:05:38.639 and maybe make them cheap. 115 00:05:38.839 --> 00:05:42.000 That is the long term vision. But making a standard 116 00:05:42.000 --> 00:05:45.240 transistor is very very different from making a quantum dot 117 00:05:45.279 --> 00:05:49.000 for a quibit. How so, well, in a transistor, you're 118 00:05:49.120 --> 00:05:52.839 essentially just creating a switch for a current. You're controlling 119 00:05:52.879 --> 00:05:55.600 a river of electrons flowing through a channel. 120 00:05:55.839 --> 00:05:58.360 On or off a whole crowd of them, a whole crowd. 121 00:05:58.480 --> 00:06:01.639 But in a quantum dot, you need to isolate one 122 00:06:02.199 --> 00:06:05.759 single electron from that river, just one, just one, and 123 00:06:05.800 --> 00:06:09.519 you need to hold it there very gently without disturbing 124 00:06:09.560 --> 00:06:11.079 its fragile quantum state. 125 00:06:11.199 --> 00:06:13.199 Okay, so how do you build this trap? You can't 126 00:06:13.240 --> 00:06:14.959 exactly use tiny tweezers. 127 00:06:15.199 --> 00:06:17.639 No, you use electric fields. It's actually quite clever. You 128 00:06:17.720 --> 00:06:22.079 fabricate tiny metal gates electrodes on top of the silicon. 129 00:06:21.720 --> 00:06:23.560 Wafer, like the gates in a normal transistor. 130 00:06:23.759 --> 00:06:26.600 Very similar, but the arrangement is different. When you apply 131 00:06:26.639 --> 00:06:30.279 a negative voltage to these metal gates, they create an 132 00:06:30.360 --> 00:06:34.759 electric field that repels the electrons and the silicon underneath. 133 00:06:34.360 --> 00:06:35.360 So you're pushing them away. 134 00:06:35.480 --> 00:06:38.759 You're pushing them away and by arranging these gates in 135 00:06:38.800 --> 00:06:41.480 a very specific pattern, like a little corral, you can 136 00:06:41.519 --> 00:06:45.839 create a small island, a box of low energy surrounded 137 00:06:45.839 --> 00:06:48.839 by high energy walls. An electron can get trapped in 138 00:06:48.839 --> 00:06:49.360 that box. 139 00:06:49.519 --> 00:06:52.519 So it's like using invisible force fields to build a 140 00:06:52.600 --> 00:06:54.240 jail cell for an electron. 141 00:06:53.959 --> 00:06:56.720 A very very small jail cell. We are talking about 142 00:06:56.800 --> 00:06:59.600 structures that are just tens of nanometers across. 143 00:06:59.639 --> 00:07:01.360 And once the electron is in there, what are we 144 00:07:01.439 --> 00:07:04.800 actually using? As the bit in my laptop, it's voltage 145 00:07:04.800 --> 00:07:07.560 being high or low that represents a one or a zero. 146 00:07:07.920 --> 00:07:08.600 What is it here? 147 00:07:08.920 --> 00:07:12.600 Here? We are using a fundamental quantum property of the electron. 148 00:07:12.959 --> 00:07:14.680 We're using its spin spin. 149 00:07:15.040 --> 00:07:17.000 This is one of those chronum terms that everyone uses, 150 00:07:17.040 --> 00:07:20.360 but it's always a bit fuzzy. Is the electron actually 151 00:07:20.399 --> 00:07:21.720 spinning like a tiny top? 152 00:07:22.079 --> 00:07:24.959 Not literally? No, that's just a helpful classical analogy. And 153 00:07:25.040 --> 00:07:28.079 electron is a point particle. It has no physical size 154 00:07:28.079 --> 00:07:28.800 to spin around. 155 00:07:28.920 --> 00:07:29.680 So what is it? Then? 156 00:07:29.920 --> 00:07:32.920 It's an intrinsic property like its charge or its mass. 157 00:07:33.160 --> 00:07:37.040 It has an intrinsic angular momentum and an associated magnetic 158 00:07:37.079 --> 00:07:40.360 moment It behaves as if it were a tiny spinning bar. 159 00:07:40.319 --> 00:07:42.519 Magnet a tiny magnet eject and. 160 00:07:42.439 --> 00:07:46.639 That tiny magnet can point in different directions. For our purposes, 161 00:07:46.680 --> 00:07:50.240 we care about whether it's pointing up or down relative 162 00:07:50.319 --> 00:07:52.199 to an external magnetic field we apply. 163 00:07:52.360 --> 00:07:54.800 Okay, so spin up is a one and spin down 164 00:07:54.920 --> 00:07:57.240 is a zero. Is that the basic idea. 165 00:07:57.079 --> 00:07:59.000 That is the basis of it. Yes, that's how you 166 00:07:59.040 --> 00:08:02.920 define your computationational lysis states. But because it's a quantum system, 167 00:08:02.920 --> 00:08:05.240 it's not that simple. It's not that simple. It can 168 00:08:05.279 --> 00:08:08.600 also exist in a superposition of both up and down 169 00:08:08.680 --> 00:08:11.399 at the same time until you measure it. That's where 170 00:08:11.399 --> 00:08:12.920 the real power comes from, right. 171 00:08:12.759 --> 00:08:16.600 The classic quantum weirdness. So, just to recap, we have 172 00:08:16.680 --> 00:08:20.360 a silicon chip. We have tiny electric gates creating a trap. 173 00:08:20.560 --> 00:08:23.959 We catch one single electron. We use its magnetic spin 174 00:08:24.079 --> 00:08:25.319 as our data storage. 175 00:08:25.439 --> 00:08:28.399 Correct. But here's the problem, the core vulnerability of this 176 00:08:28.439 --> 00:08:32.120 whole approach. That spin state is incredibly fragile. Well canness 177 00:08:32.120 --> 00:08:35.480 it up almost anything. The biggest enemy is thermal energy 178 00:08:35.720 --> 00:08:39.279 just heat. Heat is just atoms and electrons jiggling around. 179 00:08:39.440 --> 00:08:42.120 If the environment is too hot, the electron shakes, it 180 00:08:42.159 --> 00:08:44.840 interacts with the lattice of silicon atoms around it, and 181 00:08:44.879 --> 00:08:48.879 the spin state just flips randomly. Your data is lost instantly, 182 00:08:49.519 --> 00:08:52.919 your one becomes a zero, or your superposition just collapses. 183 00:08:52.960 --> 00:08:56.240 The information is gone. This is called decoherence. 184 00:08:56.600 --> 00:08:57.559 So you have to freeze it. 185 00:08:57.639 --> 00:09:00.600 You have to freeze it to near absolute zero. These 186 00:09:00.679 --> 00:09:03.000 chips don't just sit on a lab bench. They live 187 00:09:03.000 --> 00:09:06.840 inside a complex machine called a dilution refrigerator, operating at 188 00:09:06.879 --> 00:09:08.679 temperatures in the millikelvin range. 189 00:09:08.879 --> 00:09:09.679 How cold is that? 190 00:09:09.960 --> 00:09:12.879 It is significantly colder there than in deep space. 191 00:09:13.080 --> 00:09:17.320 So I should picture this a giant humming silver cylinder 192 00:09:17.480 --> 00:09:20.679 in a lab. Inside it, there are layers and layers 193 00:09:20.720 --> 00:09:24.720 of gold and copper plates that get progressively colder and colder, 194 00:09:25.000 --> 00:09:28.080 and at the very bottom, in the deepest cold sits 195 00:09:28.200 --> 00:09:32.600 this tiny silicon chip with these nanometer scale traps. 196 00:09:32.759 --> 00:09:34.679 That is the exact picture, and that is where the 197 00:09:34.759 --> 00:09:37.799 racing team is working. They aren't just looking at one dot. 198 00:09:37.840 --> 00:09:42.279 Though one quibbit is a science experiment, a computer needs many, and. 199 00:09:42.159 --> 00:09:44.480 This is where we hit the scaling problem, the crowded house, 200 00:09:44.519 --> 00:09:45.600 as we called it. Right. 201 00:09:45.679 --> 00:09:49.399 To perform calculations, to run in algorithm, quibits need to 202 00:09:49.440 --> 00:09:51.639 interact with each other in a controlled way. You need 203 00:09:51.720 --> 00:09:53.320 to be able to entangle them. 204 00:09:53.200 --> 00:09:54.960 Which means they can't be miles apart. 205 00:09:55.159 --> 00:09:56.799 They need to be close, very close. 206 00:09:56.879 --> 00:09:58.039 How close were we're talking. 207 00:09:57.960 --> 00:10:01.000 In the device and this study, the which is the 208 00:10:01.039 --> 00:10:03.440 distance from the center of one quantum dot to the 209 00:10:03.480 --> 00:10:05.679 center of the next, is about one hundred to one 210 00:10:05.720 --> 00:10:06.879 hundred and fifty nanometers. 211 00:10:07.039 --> 00:10:08.600 That is incredibly tight. 212 00:10:08.759 --> 00:10:11.559 It is extremely tight. And when you pack charged particles 213 00:10:11.600 --> 00:10:15.639 that close together, you run into a fundamental force of nature, 214 00:10:15.840 --> 00:10:17.000 the kulum interaction. 215 00:10:17.279 --> 00:10:18.559 Remind us what that is, It's. 216 00:10:18.440 --> 00:10:23.519 The most basic electric force. Like charges repel, opposite charges attract. 217 00:10:23.639 --> 00:10:27.559 So two electrons, two electrons are both negatively charged. They 218 00:10:27.639 --> 00:10:30.320 hate each other. They are constantly trying to push each 219 00:10:30.320 --> 00:10:30.879 other away. 220 00:10:31.000 --> 00:10:33.120 So i have two of these quantum dots right next 221 00:10:33.120 --> 00:10:36.320 to each other, and I've got one electron in each one. 222 00:10:36.440 --> 00:10:40.559 They're constantly pushing against the walls of their little jail cells, 223 00:10:40.679 --> 00:10:42.039 trying to get away from their neighbor. 224 00:10:42.240 --> 00:10:45.799 Ideally, the electric field walls of the trap are strong 225 00:10:45.879 --> 00:10:48.440 enough to hold them in place, but they still feel 226 00:10:48.440 --> 00:10:51.039 each other's presence. The force is still there. Okay, So 227 00:10:51.279 --> 00:10:54.559 if the electron in Dot A shifts its position slightly, 228 00:10:54.799 --> 00:10:57.200 and it can. It has a little bit of wiggle room, 229 00:10:57.360 --> 00:10:58.480 maybe just by a nanometer. 230 00:10:58.840 --> 00:11:01.720 The electron in dot beef that the electron. 231 00:11:01.279 --> 00:11:03.639 In dot B feels a change in the repulsive force. 232 00:11:04.000 --> 00:11:06.440 The push gets a little stronger or a little weaker. 233 00:11:06.639 --> 00:11:08.720 It's like if you are standing in a small room 234 00:11:09.080 --> 00:11:11.919 with a powerful magnet strapped to your chest and someone 235 00:11:11.919 --> 00:11:14.279 in the next room had one too, even if there's 236 00:11:14.320 --> 00:11:16.600 a wall, if they take a step towards that wall, 237 00:11:16.759 --> 00:11:17.600 you feel a push. 238 00:11:17.799 --> 00:11:21.639 That is a perfect analogy, and this brings us right 239 00:11:21.679 --> 00:11:25.159 to the core of the Reichen discovery. Takashi Kobayashi and 240 00:11:25.200 --> 00:11:28.960 his team, we're investigating this exact scenario. They wanted to know, 241 00:11:29.720 --> 00:11:33.279 does that push from the neighbor mess up the information 242 00:11:33.440 --> 00:11:35.600 stored in the spin of our kubit. 243 00:11:35.320 --> 00:11:38.279 And the answer I'm going to assume is yes, otherwise 244 00:11:38.279 --> 00:11:39.720 we wouldn't be doing this whole conversation. 245 00:11:39.919 --> 00:11:43.159 The answer is a definitive yes. But the mechanism, the 246 00:11:43.360 --> 00:11:46.480 how is the really fascinating part because it's not a 247 00:11:46.519 --> 00:11:49.799 direct effect. It involves a component we haven't even mentioned yet. Oh, 248 00:11:49.840 --> 00:11:50.799 the micro magnet. 249 00:11:50.919 --> 00:11:54.159 The micromagnet. Okay, wait, let's back up. We have electric 250 00:11:54.200 --> 00:11:57.840 gates to trap the electron. We have the electron itself 251 00:11:57.879 --> 00:12:01.320 acting like a tiny magnet with its spin, And why 252 00:12:01.360 --> 00:12:03.000 on earth do we need another magnet. 253 00:12:03.279 --> 00:12:05.440 This is a great question, and it gets right to 254 00:12:05.480 --> 00:12:08.480 the heart of why silicon spin quibets are so hard 255 00:12:08.519 --> 00:12:11.639 to build and control. Remember, what we want to do 256 00:12:11.759 --> 00:12:14.440 is control the spin of the electron. 257 00:12:14.120 --> 00:12:16.480 Right flip it from up to down, or put it 258 00:12:16.519 --> 00:12:18.360 in this superposition exactly. 259 00:12:18.799 --> 00:12:22.279 Now, spin responds to magnetic fields, so the obvious way 260 00:12:22.320 --> 00:12:24.080 to control it would be to zap it with a 261 00:12:24.080 --> 00:12:25.240 little magnetic pulse. 262 00:12:25.480 --> 00:12:26.480 Seems simple enough. 263 00:12:26.679 --> 00:12:29.360 It is if you only have one quibbit. But remember 264 00:12:29.360 --> 00:12:31.360 we're in a crowded house. If you want to build 265 00:12:31.360 --> 00:12:34.840 a real computer, you have thousands, maybe millions, of these 266 00:12:34.919 --> 00:12:36.320 dots packed together, and if I. 267 00:12:36.279 --> 00:12:39.240 Turn on a magnet, everything nearby feels it. It's hard 268 00:12:39.279 --> 00:12:42.559 to target just one dot without disturbing all its neighbors. 269 00:12:42.679 --> 00:12:46.559 Precisely, it's called addressability, and it's a huge problem. You'd 270 00:12:46.559 --> 00:12:50.080 have magnetic cross stock everywhere. So the engineer's thought, is 271 00:12:50.080 --> 00:12:53.039 there a better way. We are very good at generating 272 00:12:53.080 --> 00:12:56.720 localized electric fields. We can switch voltages on those tiny 273 00:12:56.759 --> 00:12:59.759 gates incredibly fast and with pinpoint precision. 274 00:13:00.039 --> 00:13:03.440 So they wanted to control the spin using electricity instead 275 00:13:03.440 --> 00:13:04.200 of magnetism. 276 00:13:04.279 --> 00:13:04.919 That was the goal. 277 00:13:05.000 --> 00:13:06.960 But can you even do that? I thought you said 278 00:13:06.960 --> 00:13:08.440 spin doesn't talk to electricity. 279 00:13:08.639 --> 00:13:12.159 It doesn't, not directly. You need a translator. You need 280 00:13:12.200 --> 00:13:16.600 a physical mechanism that couples the electrons position which electricity controls, 281 00:13:16.840 --> 00:13:20.039 to its spin state. This is a real physical effect 282 00:13:20.080 --> 00:13:24.639 called spin orbit coupling. Okay, in some materials like gallium arsenide. 283 00:13:24.799 --> 00:13:28.799 This effect happens naturally and is quite strong. In silicon, 284 00:13:29.039 --> 00:13:32.000 which we want to use for manufacturing reasons, it's naturally 285 00:13:32.200 --> 00:13:32.960 very very weak. 286 00:13:33.039 --> 00:13:33.879 So we have to fake it. 287 00:13:34.000 --> 00:13:36.600 We have to engineer it. We create something called synthetic 288 00:13:36.720 --> 00:13:37.720 spin orbit coupling. 289 00:13:37.879 --> 00:13:39.320 Synthetic spin orbit coupling. 290 00:13:39.519 --> 00:13:42.279 That sounds expensive, it's clever engineering. This is why they 291 00:13:42.320 --> 00:13:46.159 install the micromagnet. They physically place a tiny strip of 292 00:13:46.240 --> 00:13:49.799 magnetic material like cobalt, right on top of the quantum dots. 293 00:13:50.279 --> 00:13:53.559 But this magnet isn't designed to create a uniform field. 294 00:13:53.600 --> 00:13:56.360 That's the key. It's designed to create a gradient. 295 00:13:56.559 --> 00:13:58.000 A gradient, what do you mean by that? So the 296 00:13:58.000 --> 00:14:00.519 magnetic field is stronger on one side of the day the. 297 00:14:00.480 --> 00:14:04.039 Other exactly that. Imagine the quantum dot is a small room. 298 00:14:04.279 --> 00:14:06.759 The magnetic field at the left wall is let's say, 299 00:14:06.840 --> 00:14:09.919 one hundred units strong. The magnetic field at the right 300 00:14:09.919 --> 00:14:12.919 wall is one hundred and five units strong. It's a slope. 301 00:14:12.960 --> 00:14:13.679 It's not flat. 302 00:14:14.000 --> 00:14:17.320 Okay, I have a magnetic slope across my tiny room. 303 00:14:17.600 --> 00:14:18.240 Why no? 304 00:14:18.440 --> 00:14:22.600 Remember the frequency at which the electrons spin processes, the 305 00:14:22.639 --> 00:14:25.960 speed at which it wobbles like a top, depends directly 306 00:14:26.000 --> 00:14:28.240 on the strength of the magnetic field it is sitting in. 307 00:14:28.559 --> 00:14:30.240 This is called the Larmer frequency. 308 00:14:30.440 --> 00:14:32.159 So if the electron is sitting on the left side 309 00:14:32.200 --> 00:14:35.120 of the room, it spin wobbles it's speed one hundred. 310 00:14:35.360 --> 00:14:37.320 If a moving to the right side, it wobbles its 311 00:14:37.320 --> 00:14:38.159 speed one oh five. 312 00:14:38.240 --> 00:14:40.879 Precisely. Now you see the trick. We can use our 313 00:14:40.919 --> 00:14:44.559 electric gates to gently nudge the electrons position. If we 314 00:14:44.600 --> 00:14:46.519 apply a little voltage, we can push it from the 315 00:14:46.559 --> 00:14:47.919 left side of the dot to the right. 316 00:14:48.039 --> 00:14:50.480 Huh. And by pushing it we force it to move 317 00:14:50.519 --> 00:14:52.639 into a stronger part of the magnetic. 318 00:14:52.240 --> 00:14:54.240 Field, which changes its spin speed. 319 00:14:54.399 --> 00:14:56.480 Uh huh. So we are using electricity to move it, 320 00:14:56.919 --> 00:15:00.519 but it's the motion through that pre engineered magnetic gradient 321 00:15:00.759 --> 00:15:02.039 that actually changes the spin. 322 00:15:02.200 --> 00:15:05.000 You've got it. The micromagnet acts as a translator. It 323 00:15:05.039 --> 00:15:07.600 converts an electric signal, which is a change in position, 324 00:15:07.840 --> 00:15:11.039 into a magnetic control, which is a change in spin rotation. 325 00:15:11.159 --> 00:15:11.480 Skeeed. 326 00:15:12.159 --> 00:15:14.879 It acts as a lever. It makes the electrons spin 327 00:15:15.200 --> 00:15:16.879 highly sensitive to its physical location. 328 00:15:17.279 --> 00:15:21.240 That sounds brilliant. It's a really clever workaround problem solved right. 329 00:15:21.279 --> 00:15:24.240 It is brilliant. It's a technique called electric dipole spin 330 00:15:24.360 --> 00:15:29.000 resonance or EDSR, and it allows for very fast, localized 331 00:15:29.039 --> 00:15:30.720 control of individual quibbits. 332 00:15:31.240 --> 00:15:32.720 But there's always a butt. 333 00:15:32.960 --> 00:15:34.799 There is no free lunch in physics. 334 00:15:34.960 --> 00:15:36.679 I knew it. Here comes the bill. 335 00:15:36.960 --> 00:15:40.399 By making the spin exquisitely sensitive to its own position, 336 00:15:40.799 --> 00:15:44.120 you have also made it exquisitely sensitive to anything else 337 00:15:44.200 --> 00:15:45.279 that might shift. 338 00:15:45.000 --> 00:15:47.919 Its position, like the neighbor pushing on the wall. 339 00:15:47.960 --> 00:15:50.080 Like the neighbor. Let's go back to our crowded house. 340 00:15:50.120 --> 00:15:52.399 We have two dots Dot A and Dot B side 341 00:15:52.399 --> 00:15:56.399 by side. Each one is sitting under this micromagnet gradient. Okay, 342 00:15:56.480 --> 00:15:59.600 Now suppose the electron in the neighboring dot Dot A 343 00:16:00.000 --> 00:16:03.399 move Maybe we're performing an operation on it, or maybe 344 00:16:03.399 --> 00:16:05.879 it just jiggles because of some background charge noise. 345 00:16:05.919 --> 00:16:07.039 The neighbor moves the sofa. 346 00:16:07.120 --> 00:16:09.799 The neighbor moves the sofa. When that electron in Dot 347 00:16:09.799 --> 00:16:13.480 A moves, the kolum repulsion it exerts on our electron 348 00:16:13.519 --> 00:16:15.000 in Dot B changes. 349 00:16:14.720 --> 00:16:16.519 The push gets stronger or weaker. 350 00:16:16.840 --> 00:16:19.960 And because our electron in Dot B is sitting in 351 00:16:20.000 --> 00:16:24.320 this carefully balanced electric trap, that tiny change in push 352 00:16:24.399 --> 00:16:27.600 from the neighbor is enough to shift its positions slightly. 353 00:16:28.000 --> 00:16:31.320 And because of the micromagnet ingredient, when our electron. 354 00:16:30.919 --> 00:16:34.080 Moves, it moves into a different magnetic field strength, and 355 00:16:34.159 --> 00:16:36.799 therefore it spin frequency changes bingo. 356 00:16:37.759 --> 00:16:40.759 The neighbor's motion has changed the speed at which our 357 00:16:40.840 --> 00:16:44.480 quibod is spinning. This is the crosstock. This is what 358 00:16:44.519 --> 00:16:48.399 the Reichen paper calls a charge induced spin frequency shift. 359 00:16:48.600 --> 00:16:49.639 That's it in a nutshell. 360 00:16:49.720 --> 00:16:52.399 So the micromagnet is a complete double edged sword. It 361 00:16:52.440 --> 00:16:54.960 gives us the precise control we need, but at the 362 00:16:55.000 --> 00:16:57.960 same time it acts as an amplifier for the noise 363 00:16:58.039 --> 00:16:58.960 coming from the neighbors. 364 00:16:59.120 --> 00:17:01.879 That is the crux of the problem they investigated. You 365 00:17:01.919 --> 00:17:04.920 built a system that is hyper sensitive to motion, so 366 00:17:04.960 --> 00:17:07.519 you could control it with electricity, but now it's hyper 367 00:17:07.519 --> 00:17:09.920 sensitive to the unwonted motion of the guy next door. 368 00:17:10.079 --> 00:17:13.599 Okay, so this sounds bad in theory. But was it 369 00:17:13.680 --> 00:17:16.200 just a theoretical worry or is it actually messing things 370 00:17:16.240 --> 00:17:17.079 up in a real device. 371 00:17:17.599 --> 00:17:21.119 That is the billion dollar question, and it's what Kobayashi's 372 00:17:21.160 --> 00:17:24.039 team went to find out. They didn't just model this 373 00:17:24.160 --> 00:17:26.880 on a computer. They built the device, They cooled it 374 00:17:26.920 --> 00:17:29.960 down to one hundred miliatelvin, and they measured it directly. 375 00:17:30.119 --> 00:17:30.599 Wow. 376 00:17:30.759 --> 00:17:33.160 And the way they measured it is fascinating in itself. 377 00:17:33.240 --> 00:17:34.839 I was going to ask, how do you measure a 378 00:17:34.880 --> 00:17:38.519 tiny shift in the wobble frequency of a single electron 379 00:17:38.880 --> 00:17:40.480 trapped in a piece of silicon. 380 00:17:40.759 --> 00:17:43.799 You use a very sensitive technique called Ramsey interferometry. 381 00:17:43.920 --> 00:17:47.279 Ramseiator ferometry. Is that related to Ramsey kitchen nightmares? 382 00:17:47.359 --> 00:17:49.559 No relation, though it can be a nightmare if you 383 00:17:49.599 --> 00:17:52.720 don't get the calibration right. It works a bit like this. 384 00:17:53.240 --> 00:17:55.880 You start with the electron spin pointing up. Let's call 385 00:17:55.920 --> 00:17:58.000 that our starting line. Okay, You hit it with a 386 00:17:58.039 --> 00:18:02.359 precisely timed microwave pul ulse that puts it into a 387 00:18:02.359 --> 00:18:06.400 perfect superposition half up, half down. Now you can picture 388 00:18:06.400 --> 00:18:09.319 its state as a clockhand pointing, say at three o'clock. 389 00:18:09.400 --> 00:18:12.160 Got it, and then you just wait. You let it evolve. 390 00:18:12.519 --> 00:18:15.839 That clockhand starts sweeping around at its natural frequency, the 391 00:18:15.920 --> 00:18:16.799 Larmer frequency. 392 00:18:16.839 --> 00:18:17.359 Tick, tick tick. 393 00:18:17.640 --> 00:18:20.559 You let it tick for a very specific amount of time. 394 00:18:21.160 --> 00:18:24.759 Then you hit it with a second identical microwave pulse. 395 00:18:25.440 --> 00:18:27.799 If the frequency was exactly what you thought it was, 396 00:18:28.240 --> 00:18:30.799 this second pulse will push the clock hand all the 397 00:18:30.799 --> 00:18:32.960 way around to point down, so. 398 00:18:32.920 --> 00:18:34.880 You know exactly where it should end up. It's like 399 00:18:34.960 --> 00:18:37.519 checking your watch against a master clock exactly. 400 00:18:37.880 --> 00:18:41.279 But what if the frequency has shifted during that waiting time. 401 00:18:41.559 --> 00:18:44.559